1. Field of the Invention
The present invention relates, in general, to a plasma display panel (hereinafter referred to as "PDP") and, more particularly, to an alternative current PDP employing a dielectric layer.
2. Description of the Prior Art
As is well known, a PDP is a device which displays pictures by exploiting so-called "gas discharge phenomenon", an electric discharge occurring across two apart points in a gas space when they are applied with an electric potential larger than a critical value.
The PDP of the simplest structure is of direct current type, in which sets of parallel electrodes at right angles to each other are deposited on two plates, with the very small space filled with a discharge gas. In the DC PDP, a pixel, which is defined by each intersection of two selected electrodes, is energized to produce a gas discharge forming one element of a dot-matrix display.
However, DC PDPs are incapable of high intensity expressions and thus, it is virtually impossible for DC PDPs to display a dynamic image of high resolution. Recently, many improved PDPs have been developed and now, some are being put into practice.
In order to better understand the background of the invention, a description will be given of a conventional technique, in conjunction with some drawings.
Largely, the improved PDPs are based on the principle of such an alternative current PDP as shown in FIG. 1. Two plates P1 and P2 are opposite to each other with a plurality of parallel electrodes E1 on the plate P1 being across a plurality of parallel electrodes E2 on the plate P2. The space sealed by a side wall W is filled with a discharge gas. A fluorescent layer F is formed on the side of the plate P2 in the sealed space in order to increase luminosity and express desired colors. Opposite to the fluorescent layer F, a dielectric layer D is laminated on the electrode E1, through which a discharge occurs and wall charges are formed. Thus, the dielectric layer D confers on the AC PDP a high responsivity and a high intensity when discharging and allows the AC PDP to maintain the discharging, so that a high luminescence brightness can be established. The character B in FIG. 1 stands for barriers for compartmenting the pixels.
Typically, the dielectric layer D is made by printing and calcining glass. This conventional dielectric layer D is apt to frequently cause so-called "ion bombardment". That is, the gas plasma leaks through the fractures formed in the electrodes E1, damaging the electrodes E1.
To overcome this disadvantage, the dielectric layer D is supplemented with a highly dense and uniform overcoat layer V by deposition. Generally, a MgO layer is vapor-deposited as the overcoat layer V.
Referring to FIG. 2, there are illustrated the processes of fabricating such an AC PDP.
First, as shown in FIG. 2A, a plurality of parallel electrodes E1 are formed on a plate P1 (front plate) to be applied with a dielectric layer D.
Next, a glass material is entirely coated over the electrodes E1 by printing and then, subjected to sintering, to give the dielectric layer D.
FIG. 2C is a cross section after MgO is deposited over the dielectric layer D by sputtering, to give an overcoat layer D. With this, the plate P1 is completed.
Separately, a plate P2 (backing plate) is prepared in which a plurality of parallel electrodes E2 are arranged and a fluorescent layer F and barriers are provided thereon. The two plates P1 and P2 are sealed by a sealant W' in such a way that the two sets of the electrodes E1 and E2 are opposite and at right angles to each other, to produce a PDP, as shown in FIG. 2D. For the sealing, a sealant W' is coated on a predetermined region of the plate P2 and the other plate P1 is placed thereon. These integrated plates are brought into a high temperature atmosphere to sinter the sealant W'. In result, a side wall W is formed, bond-sealing the two plates P1 and P2 to each other.
The completed PDPs are brought to market after aging and performance testing. A significant quantity of PDPs have defectives in entirety or locally and thus are wasted. Even after being sold, they are frequently returned defective before the end of the guarantee period.
The causes of the defectives come, in part, from the process of printing or sintering. And, most of the defectives are attributed to a pollution of discharge gas or a local damage of the electrodes. In the latter case, cracks in the overcoat layer V play a critical role. That is, through the cracks, Pb is diffused from the dielectric layer D into the discharge gas and the discharge plasma leaks to damage the electrodes E1.
It was found that the occurrence of cracks in the overcoat layer V is chiefly made after the sealing of the panel. Conventionally, this problem was believed to be attributed to the thermal properties of the overcoat layer V and thus, there have been tried to variously change the heating atmosphere for the sealing process into, for example, more gradually slow heating or cooling atmospheres. However, no particular improvements have been obtained.
It was also found that, although the sealant W' had a sintering temperature, that is, a softening temperature ranging from approximately 400 to 450.degree. C. and MgO, the metal oxide constituent for the overcoat layer V, had a melting temperature of around 1,000.degree. C., the thermal resistance temperature at which no crack occurred in the overcoat layer consisting of MgO, was only approximately 400.degree. C. Further, since MgO was selected by virtue of its showing a similar coefficient of thermal expansion to that of the glassy dielectric layer, the cause of the cracking in the overcoat layer V has not been clear.